First Earth-sized planets spotted

The planets may once have harboured conditions favourable to life

Related Stories

Astronomers have detected the first Earth-sized planets, which are orbiting a star similar to our own Sun.

In the distant past they may have been able to support life and one of them may have had conditions similar to our own planet - but they now lie outside their star's "habitable zone".

They have described their findings as the most important planets ever discovered outside our Solar System.

Details of the discovery are outlined in the journal Nature.

Dr Francois Fressin, of the Harvard-Smithsonian Centre for Astrophysics in Cambridge, US, who led the research, said that the discovery was the beginning of a "new era" of discovery of many more planets similar to our own.

Both of the newly discovered planets are now thought to be too hot to be capable of supporting life.

But according to Dr Fressin, the planets were once further from their star and cool enough for liquid water to exist on their surface, which is a necessary condition for life.

"We know that these two planets may have migrated closer to their Sun," he told BBC News. "(The larger of the two) might have been an Earth twin in the past. It has the same size as Earth and in the past it could have had the same temperature".

Rock and a hard place

One of the planets, named Kepler 20f, is almost exactly the size of the Earth. Kepler 20e is slightly smaller at 0.87 times the radius of Earth and is closer to its star than 20f.

Kepler Space Telescope

Infographic (BBC)
  • Stares fixedly at a patch corresponding to 1/400th of the sky
  • Looks at more than 155,000 stars
  • Has so far found 2,326 candidate planets
  • Among them are 207 Earth-sized planets, 10 of which are in the "habitable zone" where liquid water can exist

They are both much closer to their star than the Earth is to the Sun and so they complete an orbit much more quickly: 20e circles its star in just six days, 20f completes an orbit in 20 days whereas the Earth takes a year.

Dr Fressin says that the planets' composition may be similar to Earth's with a third of it consisting of iron core and the remainder consisting of a silicate mantle. He also believes that the outer planet (Kepler 20f) may even have developed a thick, water vapour atmosphere.

The discovery is important because it is the first confirmation that planets the size of Earth and smaller exist outside our Solar System. It also shows that the Kepler Space Telescope is capable of detecting relatively small planets around stars that are thousands of light-years away.

The telescope has spotted more than 2,300 planet candidates, which must then be confirmed by other telescopes in space and back on Earth.

Of the 36 of these that have now been confirmed, only Kepler 20e is smaller than Earth.

Up until now, the most significant discovery, also by a group including Dr Fressin, was of a planet nearly two-and-a-half times the size of Earth that lay in the so-called "Goldilocks zone".

This is the region around a star where it is neither too hot, nor too cold, but just right for liquid water and therefore life to exist on the planet.

But Dr Fressin believes that the two new planets are a much more important discovery.

The telescope is scanning 150,000 stars and Professor Andrew Coates of the Mullard Space Science Laboratory in Surrey believes that they will soon find a planet that is not only the size of Earth, but is also in the Goldilocks zone - and perhaps even Earth-like in its composition and atmosphere.

"With every new discovery we're getting closer to the 'holy grail' of an Earth-like planet around a Sun-like star," he said.

Schematic of Kepler 20 planet sizes Simulations of the planets' appearances show their relative sizes

Follow Pallab on Twitter

More on This Story

Related Stories

The BBC is not responsible for the content of external Internet sites

More Science & Environment stories


Features & Analysis

  • BeefaloBeefalo hunt

    The hybrid animal causing havoc in the Grand Canyon

  • Actor Jackie Chan gestures as he stands on the set of his new movie 'Around the World in 80 Days' on 6 May, 2003 in Berlin, GermanyDuang duang duang

    How a new word 'broke the Chinese internet'

  • Sound of Music PosterFar from a flop

    Even Sound of Music film crew surprised by success

  • Don Roberto Placa Quiet Don

    The world's worst interview - with one of the loneliest men on Earth

Elsewhere on the BBC

  • KnucklesGood or bad?

    For many it can be very satisfying to 'crack' the bones in your hand, but is it bad for you?


  • 3D model of Christ the Redeemer statueClick Watch

    Using drones to 3D map the famous Brazilian landmark Christ the Redeemer

Try our new site and tell us what you think. Learn more
Take me there

Copyright © 2015 BBC. The BBC is not responsible for the content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.